1. Field of the Invention
The invention relates to a digital holography device, for example used for the 3D mapping of objects, and especially moving objects, for example under vibration, and also for the measurement of optical components or the study of scattering media.
2. Description of the Prior Art
Conventional holography, invented by Dennis Gabor in 1948, underwent development particularly in the 1960s with the appearance of lasers. It is a method by which it is possible to reconstruct a 3D image of any object from a recording made on a photographic plate called a hologram, without the assistance of any objective. Today, the applications of conventional holography are numerous. A real 3D image that is obtained during restitution gives an impressive effect of relief. It is thus possible to obtain beautiful holograms of art objects, even fairly big ones. In science and industry, holographic interferometry enables the study of changes in the shape of a variety of elements, under different forces, such as for example the study of fluid flows in wind tunnels.
FIGS. 1A and 1B show the recording and restitution of a conventional hologram of a source point OBJ. During the recording (FIG. 1A) a recording is made, on a photographic plate HOLO, of the variations in intensity due to the interferences of a reference wave WREF, for example a plane wave, with a coherent wave WS scattered by the object. After development according to ordinary methods of photography, the photographic plate constitutes the hologram. During the restitution (FIG. 1B), the hologram HOLO is illuminated by the reference wave WREF and produces two waves of diffractive light, one wave WOBJ that reconstitutes a virtual 3D image of the object and one conjugate wave WCONJ that forms a real image of the object point OJB, which is a parasitic image to be eliminated. One way to separate the diffractive waves then consists of the use of a thick material as the holographic emulsion. Thus, a so-called volume hologram is made in which the interferences occur throughout the thickness of the material. At restitution, the diffraction obeys the Bragg condition and only the object wave is diffracted.
However, while excellent-quality images can be reconstituted through a very high resolution of the photographic emulsions, there is no direct access to the information registered. This limits the applications of conventional holography to qualitative observations. It is not possible for example to make a 3D mapping of an object or have access to quantitative measurements of the photometry parameters. To overcome this drawback and obtain information that can be quantitatively exploited, new techniques of holography have appeared. These techniques replace the photographic film by a 2D optoelectronic detector, for example a CCD camera. In digital holography devices, the interferences between the reference wave and a wave scattered by the object (the signal wave) are recorded in the plane of the detector. These two waves have come from the same laser source in order to meet the conditions of coherence. The acquisition of the interference signal by the detector makes it possible to digitize the information and determine sampled values of the phase and amplitude of the scattered wave. These data elements may then be exploited to obtain quantitative results on the object.
However, just as in conventional holography on thin films, the field pattern of the computed signal wave simultaneously shows the desired order, namely the zero order, and a parasitic conjugate order. There is therefore a degree of indeterminacy in the complex amplitude of the scattered wave that is to be determined. This indeterminacy has repercussions on the quality of the results obtained by this technique. For example, the mapping of the object to be established will be determined with lack of precision.
The digital holography device according to the invention enables the full and extremely precise determination of the complex amplitude of the wave scattered by the object. This full determination is furthermore very well suited to the study of vibrating objects for example.